Skeletal Muscle
Functions
• 1. Movement - contract (get shorter)  bones
function as levers
• 2. Heat production through catabolism (
homeostasis)
• 3. Posture - muscle tone (constant, slight flexing
of muscles)
• 4. Joint stabilization
Macroscopic Structure
• 1. Belly = main part of the
muscle
• 2. Origin = attachment to
the nonmoving bone
• 3. Insertion = attachment
to the moving bone
Skeletal Muscle Organs
• 1. Components - Study drawings
(pp. 282, 283, 285) know for quiz!!!
– a. Epimysium (deep fascia) fibrous connective tissue sheath
around each muscle.
Extends past muscle as:
• 1) Tendon - connects muscle to
bone
• 2) Aponeurosis - connects
muscle to muscle as a flattened
connective tissue sheet
Skeletal Muscle Organs (continued)
• b. Perimysium connective tissue around
bundles of deep fascia
fibers (cells)
1) Fasicle = bundle of
muscle cells
2) Myofiber = muscle cell
Skeletal Muscle Organs (continued)
• c. Endomysium - around each
muscle fiber (cell)
1. Myofibril - bundle of muscle
proteins
2. Myofilament - muscle protein
(actin (thin) or myosin
(thick))
3. Sarcomere the unit of
contraction, from Z disc to Z
disc
Skeletal Muscle Organs (continued)
• 2. Nerve Supply
– a. Somatic motor neuron =
nerve cell that tells a
skeletal muscle to
contract.
– b. Motor unit = one neuron
and all of the muscle cells
it goes to.
Skeletal Muscle Organs (continued)
• c. Motor end-plate or
neuromuscular junction =
where neuron and myofiber
meet at synaptic cleft
(which is the space
between them)
• d. Acetylcholine (Ach) =
neurotransmitter
(chemical) that initiates
contraction. Broken down
by acetylcholinesterase
(Ach-ase) enzyme.
Muscle
• 1. Contracts only when
stimulated by a motor neuron.
• 2. Types of contraction:
– a. Tonic contraction = continual
partial contraction  muscle
tone and posture.
– b. Isotonic contraction = tone
stays the same, but muscle
shortens  movement.
– c. Isometric = length stays the
same, but tone increases.
– d. Twitch = quick, jerky
contraction in response to a
single stimulus (done in lab with
electricity.)
Types of contraction (continued):
• e. Summation - 2nd contraction is stronger if 2nd stimulus is
applied before relaxation of the first contraction.
• f. Treppe (unfused tetanus) (staircase phenomenon) muscle contracts more forcefully after a few contractions
(warm up exercise ex.) to a maximum.
• g. Tetanic contraction (tetanus) - from series of stimulation
with no relaxation period between. (Normal movements are
by complete tetanus.)
• h. Convulsions = abnormal, uncoordinated tetanic
contractions.
• 3. Skeletal muscle organs contract on graded strength
principle.
• 4. Skeletal muscle cells contract on the all-or-none principle
(once threshold has been reached.)
• 5. Strength of contraction depends on:
– a. Initial fiber length (Starling’s law of heart muscle) = a
stretched muscle contracts with more force.
– b. Metabolic condition (O2 and food supply)
– c. Number of fibers contracting (depends on frequency
and intensity of stimulation.)
• d. Previous contraction (warm up)
• e. Load (recruitment) - lifting a pencil compared to lifting a
book.
• f. Muscle fatigue - build up of lactic acid until muscle can no
longer contract  O2 debt.
Muscle Movement
• 1. Skeletal Muscles move by pulling on
bones.
• 2. Muscles are proximal to the part they
move.
• 3. Muscles act in groups
– a. Prime movers - produce
Work in
the movement
opposition to
– b. Antagonists - relax when
each other
the prime movers contract
– c. Synergists contract at same time as prime
movers; help stabilize the movement
4. Muscle movements
• a. Flexion / extension =
decrease / increase angle
between bones
• b. Adduction / abduction
= lower to / raise from
midline
• c. Rotation
• d. Pronation / supination
= inward / outward
turning of palm
Naming of Muscles (pp 325-26)
•
•
•
•
1. Location - temporalis • 5. Number of origins triceps
2. Shape - deltoid
3. Size - ____ maximus • 6. Location of origin
and/or insertion 4. Direction of muscle
sternocleidomastoid
fibers - _____ oblique
• 7. Action of muscle adductor _______